EP2645951B1 - Angle-variable bone screw fixation arrangement - Google Patents

Description

The invention relates to a fixation assembly for attachment by means of a bone screw. The fixation arrangement is used in particular for connection to a holding rod for an implant, but can also be used independently.

On the one hand releasable but on the other hand sufficiently fixed attachment of implants screws are often used. Depending on the application, a rigid-angle positioning between the implant and the bone screw is sufficient, but in other cases, a variable-angle (polyaxial) arrangement of the bone screws is required. Such a polyaxial bone screw for a fixation arrangement is in its basic features of the EP 0 614 649 B1 known. The bone screw is guided in a holding sleeve, which has a screw-on nut for securing. For fixing a rod of an implant, a transverse bore is provided on the retaining sleeve, through which the rod can be inserted into an intermediate region between the nut and the bone screw. By tightening the nut, the rod is clamped to the holding sleeve. At high tightening forces it can lead to twisting of the retaining sleeve, whereby the fastening security is at risk.

In an advanced design, as they are from the US 7,641,674 B2 is known, acts on a screwed-in at the rear end of the retaining sleeve pressure screw on the inserted by a transverse bore holding rod of the implant on a clamping element which has on its front side a dome-shaped recess for receiving a spherically thickened screw head of the bone screw. The clamping element is longitudinally displaceable and is pressed under the force of the pressure screw against a arranged at the front end of the retaining sleeve snap ring. So he forms an abutment for the force applied by the pressure screw pressure force. This design limits the applied pressure forces, and thus the clamping forces required to achieve an angular stability. A fixation arrangement with the features of the preamble of claim 1 is known from US 2005/154392 A1 known. The invention is based on the object, starting from the last-mentioned prior art, to provide an improved fixation arrangement which achieves a better angular fixation. The solution according to the invention lies in a fixation arrangement with the features of the independent claim. Advantageous developments are the subject of the dependent claims. In a fixation assembly comprising a holder and a bone screw receiving clamping sleeve, wherein the bone screw has a shank with a front thread and a head at the rear end, and the holder has a through hole with a receiving seat in which the clamping sleeve is pivotally mounted, and Shaft in its rear region front of the head has a thickening, the width of which increases towards the head, and the width of the clamping sleeve is greater than the width of the shaft immediately before the thickening and smaller than the largest width of the thickening. It is inventively provided that at the thickening a secondary thread is formed, which has a smaller pitch than the thread on the shaft. By "front" is meant the direction to the top and "back" the direction to the head of the bone screw.

The thickening, which increases from front to back, reaches its greatest width at the head, which is greater than the clear width of the adapter sleeve. Preferably, the thickening extends over a length which is at least two and a half times the length of the clamping sleeve. It is particularly preferred if the thickening is conical, with a cone angle of at most 15 degrees, preferably at most 5 degrees.

Under pivoting is understood here that the axis of the fixed by means of the clamping sleeve bone screw can take a freely adjustable angle to the axis of the through hole in the holder. The adjustment range is preferably at least 15 degrees in each direction about the central axis of the throughbore.

The invention is based on the idea of providing a substantial increase in the force securing the screw in its angular position by a combination of two measures. On the one hand, with the arrangement of the thickening in front of the head of the bone screw slipping out of the bone screw from the clamping sleeve is prevented even at very high power load. Thus, the bone screw can be tightened at the implantation site, whereby it moves axially forward. On the other hand, due to this axial movement, the thickening is moved into the clamping sleeve and presses against its inner wall. As a result, the clamping sleeve is elastically widened with its preferably spherical jacket and clamped with the preferably complementary shaped, d. H. spherical cap-shaped wall of the receiving seat. Depending on the type of thickening results in a considerable power transmission, at a cone angle of 5 degrees over tenfold. This can be effectively achieved a compression between the clamping sleeve and receiving seat. The bone screw is thus fixed in the desired angular position by a press fit.

Most of the thickening will have a smooth coat. This allows a low-friction tightening of the bone screw and a simplified axial relative movement to the clamping sleeve. However, it can also be provided that a secondary thread is formed on the thickening. It preferably has the same pitch as the thread on the shaft. It is particularly preferred if the secondary thread is more stable. This simplifies screwing in, especially with only a short counter thread. The mating thread can be formed in the clamping sleeve, and preferably on a rim, which rotates on the preferably cylindrical inner wall of the clamping sleeve. The radial bar prevents migration of the bone screw, and the internal thread arranged there facilitates the spread of the clamping sleeve and thus further increases the security of attachment. According to the invention, the jacket of the thickening is provided with its own thread (secondary thread). The secondary thread is formed with a pitch which is smaller than the pitch of the thread on the shaft, suitably in the range between 0.4 and 0.6. The secondary thread is therefore finer than the thread on the shaft and therefore preferably designed to be catchy. A multi-course execution should not be excluded. In front of the jacket on which the secondary thread is formed, a conical section is preferably provided, over which the width increases from the width of the shaft to the width of the jacket. The jacket itself is expediently cylindrical, but this is not mandatory. Preferably, the head is conical, in such a way that it directly adjoins the thickening and widens to the rear.
It is not essential that the thickening rises evenly out of the shaft. It can also be provided that the thickening forms a collar at its front end. It is particularly expedient to structurally combine in this case, the thickening with the head of the bone screw, in which case the outer sheath of the thickening has a conicity, which may further preferably also extend over the head.

It can further be provided that the bone screw has a conical thread with a core diameter increasing toward the head. With such a thread, a good cutting action in the bone can be achieved with high fastening reliability simple implantation technique.
The clamping sleeve is preferably designed slotted. This can be done in several ways. Thus, a running over the entire length of the clamping sleeve continuous slot can be provided. This results in a good expandability, but at the expense of a rotationally symmetrical force dissipation to the wall of the receiving seat. Alternatively (or additionally) can be provided that a plurality of unilaterally open slots are provided. Conveniently, they are open to the front side.
The clamping sleeve can be provided with a complex passage opening. This is understood to mean that the passage opening has a conical section for effecting the spreading function and a threaded section which is preferably short (at most two thread turns) and serves for clamping.

In a non-inventive embodiment, the fixation assembly comprises a screw and a holder, the screw having a threaded shaft and a ball head, and the holder comprises a holding sleeve, a clamping element and a pressure element, wherein the holding sleeve has a continuous inner bore with a front fit for the clamping element and a rear internal thread for the pressure element is provided, and wherein the tensioning element is a tensioning cage, and in the inserted state, the ball head is enclosed over the region of greatest extent and cooperates via a conical interface with the snug fit that the tensioning cage at an axial displacement compressed to the front and pressed against the ball head.

This is based on the idea to provide by a combination of two measures for a substantial increase in the screw in its angular position securing force. This is done firstly by the fact that the clamping cage encloses the ball head of the screw beyond the equator, and thus slipping out or be pushed out of the ball head excludes even at very high power load. This is combined with the intended conicity between the clamping cage and its fit in the retaining sleeve, which results in a narrowing of the clamping cage with the ball head of the screw received therein under the action of the pressure applied by the pressure element. The casing of the clamping cage enclosing the ball head is thereby pressed into the ball head with a force which is considerably increased due to wedging action, so that a compression results. Thanks to this compression combined with the protection against slipping even at very high force application a very stable angle fixation is achieved.
In order to increase the force exerted by the clamping cage pressing forces, the clamping cage is preferably provided on its jacket with the front end open slots. Thus, the clamping cage can transmit the compression forces generated by the conicity unhindered on the ball head, even if there is considerable compression, without causing undesirable internal Verzwängung in the clamping cage.
Advantageously, the pressure element acts on an outer edge of the end face of the clamping cage. By the pressure force acting on the outer edge and not on the center of the clamping cage, there is a direct force transfer to the conical interface between the outer shell of the clamping cage and the fit. Next is thus avoided that the clamping cage is pressed in the center under the load of the compressive force, whereby the ball head a it is generated from the tensioning cage ausschiebende force. The fastening security is increased.
Advantageously, the clamping cage has a radially projecting edge at its front end. This allows a pre-assembly of the clamping cage in the retaining sleeve such that the clamping cage is inserted from the front into the retaining sleeve and - after passage of the constriction at the front edge of the retaining sleeve - snaps into position, the radially projecting edge is still outside the constriction and the clamping sleeve prevents it from migrating backwards to the thread. In this unit, the ball head of the bone screw is pressed. With such a pre-assembly, the applicability for the surgeon is much easier, and also for the high risk for the patient from a possible loss of parts is banned.
The conical interface can be designed as desired. It is preferred if the tensioning cage has a conical outer edge. It can cooperate with a constriction provided at the front end of the inner bore. Such a restriction is easy to manufacture, since the remaining part of the inner bore can be made substantially cylindrical. However, it should not be ruled out that the inner bore in the region of the fit is alternatively or additionally tapered.

In one example, the interference fit is biaxial, with one segment on the sleeve and the other on the tension cage. In this case, the segment on the sleeve is the front segment, and immediately behind the segment joins the clamping cage. The rear segment on the tension cage has its greatest width at the transition to the front segment. The rear conical segment preferably closes smoothly, ie without a gap. The tensioning cage according to this example is arranged in the rear segment and acts with the hollow ball shell-like shaped segment such that the ball head, as described above, beyond the equator is included. In this case, in the variant described here, the tensioning cage itself does not extend beyond the equator. This two-piece design is easy to install and ensures a good ball head with low tolerance sensitivity.

The tensioning cage has at its rear end face preferably a groove-like concavity, which is designed to receive a holding rod. Further details of the retaining bar will be explained below. The concavity ensures a positive connection with the retaining bar and thus for a positionally accurate mounting.

The clamping cage is expediently slotted laterally in its rear region, preferably as a double slot. This spreadable tongues are created. They are expediently dimensioned so that it engages in window-like recesses on the inside of the sleeve. This results in an additional fixation of the clamping cage together with his salary bone screw, which counteracts an unintentional deposition.

Preferably, the width of the inner bore in the rear threaded area is greater than the width in the region of the fit. This allows easy pre-assembly of the clamping cage in the retaining sleeve, namely by the clamping cage is introduced from behind through the threaded portion into its position in the region of the fit. This makes it possible to choose the width of the front opening of the inner bore smaller than the width of the clamping cage. Thus, the clamping cage is secured against unintentional emigration forward.

The holding sleeve expediently has a receptacle for a holding bar of an implant. This can be a secure fixation of the implant in the areas of the bone in which the bone screw is screwed, can be achieved. Even complex implants can be secured in this way safely and stable angle. Advantageously, the receptacle is designed as a transverse bore to the inner bore holding sleeve, in the region of the transition between the rear internal thread and the front fit. In the inserted state, the rod is thus between the pressure element and the end face of the clamping cage, on which the pressure element would act directly without the rod. When inserted rod, the pressure transmission from the pressure element on the clamping cage thus takes place indirectly, namely over the cross section of the rod. This fixation of the rod is achieved relative to the fixation arrangement at the same time without further effort.

The transverse bore preferably has a non-circular cross-section. This can be achieved a positive connection between the rod and retaining sleeve. This provides additional security against unwanted distortions. In this case, the transverse bore does not necessarily have the same cross-sectional shape as the rod, but expediently, the transverse bore is elongated in the direction of the longitudinal axis of the inner bore. This provides additional adjustment and ensures a secure attachment even when the tensioning cage is positioned quite far forward (to compensate for a relatively thin ball head of the bone screw).

Preferably, the pressure element has a Sägezahngewinde, in which the rear-facing load flank is steeper than the forward facing other edge. A load flank angle of 0 ° or a negative load flank angle has proven particularly useful.

In all embodiments, the bone screw may be provided with a through bore extending along its central axis ie it is cannulated. This allows it to be used along a pushed-through guidewire, whereby a very good positioning accuracy is achieved even under difficult access conditions.

Fig. 1

eine Explosionsansicht einer Fixationsanordnung nicht gemäß der Erfindung;

Fig. 2

eine Montageansicht mit eingesetztem Stab eines Implantats;

Fig. 3

eine Querschnittansicht der Haltehülse der Fixationsanordnung;

Fig. 4

eine Seitenansicht einer Fixationsanordnung gemäß einer Ausführungsform der Erfindung;

Fig. 5

eine Detailvergrößerung aus Fig. 4;

Fig. 6

perspektivische Ansichten von Spannhülsen;

Fig. 7

Schnittdarstellungen von Spannhülsen;

Fig. 8

verschiedene Ausführungsbeispiele für Verdickungen der Knochenschraube, wobei die Darstellung in der

Fig. 8b

erfindungsgemäß ist;

Fig. 9

einen Mehrfachhalter für zwei Knochenschrauben;

Fig. 10

eine Explosionsansicht der Fixationsanordnung gemäß einer Variante der Ausführungsform der Fig. 1;

Fig. 11

eine Querschnittansicht der Haltehülse der Variante gemäß Fig. 10;

Fig. 12

eine Darstellung einer Knochenschraube für eine Variante der erfindungsgemäßen Ausführungsform; und

Fig. 13

eine Schnittdarstellung einer Spannhülse der erfindungsgemäßen Ausführungsform.

The invention will be explained below with reference to the accompanying drawing, in which an advantageous embodiment is shown. Show it:

Fig. 1

an exploded view of a fixation arrangement not according to the invention;

Fig. 2

an assembly view with inserted rod of an implant;

Fig. 3

a cross-sectional view of the retaining sleeve of the fixation assembly;

Fig. 4

a side view of a fixation arrangement according to an embodiment of the invention;

Fig. 5

an enlarged detail Fig. 4 ;

Fig. 6

perspective views of clamping sleeves;

Fig. 7

Sectional views of clamping sleeves;

Fig. 8

various embodiments for thickening of the bone screw, wherein the representation in the

Fig. 8b

is according to the invention;

Fig. 9

a multiple holder for two bone screws;

Fig. 10

an exploded view of the fixation arrangement according to a variant of the embodiment of Fig. 1 ;

Fig. 11

a cross-sectional view of the retaining sleeve of the variant according to Fig. 10 ;

Fig. 12

a representation of a bone screw for a variant of the embodiment according to the invention; and

Fig. 13

a sectional view of a clamping sleeve of the embodiment according to the invention.

The fixation assembly comprises a holder 2 for a bone screw 1. The bone screw 1 is conventionally designed with a shaft 10, on which in its front region or completely a bone thread 11 is provided. At the rear end of the shaft 10, a screw head 12 is provided, which is spherical with an equator 13 at its largest diameter. At its rear end, a star-shaped recess 14 may be provided as a receptacle for a correspondingly formed wrench.
The holder 2 comprises a sleeve 3, a clamping cage 4 and a pressure element 5. The sleeve 3 is designed like a hollow cylinder with a continuous from a rear to the front end inner opening 30. The inner opening 30 has in its rear portion an internal thread 31 and in its front region a smooth-walled fit 32 on. Here, the core diameter of the internal thread 31 is chosen so that it is a little smaller (about 1 mm) than the diameter in the region of the fit. The design of the wall in the region of the fit 32 is substantially cylindrical with a constriction 33 at the front end. It is thus formed a front mouth with a reduced diameter.
The sleeve 2 further has two diametrically opposed longitudinal slots 35. They extend from the rear end of the sleeve 3 over the entire region of the internal thread 31 into the transition region 34 between internal thread 31 and fitting fit 32. There, the slot 35 in the tangential direction to an extension.

The tensioning cage 4 has a jacket 40 provided with a conical outer surface. This is divided by six extending from the front end over about 4/5 of the length of the clamping cage slots 41 into a corresponding number of segments 42. These segments 42 are connected to one another via a retaining ring 45 which is formed in the region of the rear end face 43 and which borders a central mounting opening 44. At the front ends of the segments 42 each have an inwardly facing projection 46 is formed. The inner diameter of the clamping cage 4 is chosen so that the ball head 12 of the screw 1 is received therein with little play. The projection 46 protrudes inward to the extent that the segments 42 can only be moved over the equator 13 of the ball head 12 with elastic expansion. Thus, the recorded in the clamping cage 4 ball head 12 of the screw 1 is secured against accidental falling out.

The pressure element 5 is formed as a grub screw with a Sägezahngewinde 51 on its outer jacket. At the rear end face a star-shaped recess 54 for receiving a wrench is formed in the center. The buttress thread 51 is designed so that the rearwardly facing load-bearing threaded surface 52 is perpendicular to the lateral surface, while the forward facing other threaded surface 53 is inclined at an angle of about 70 °. With this asymmetrical sawtooth design of the thread 51, a widening of the holding sleeve 3 is avoided even under high load.

For mounting, the cage 4 is guided by elastic compression of the segments 42 from the front through the constriction 33 until the clamping cage 4 comes into abutment in the region of the interference fit 32.

In the inserted state, the front end of the segments 42 protrude with the projection 46 from the constriction forward. In the next step, the screw 1 is inserted with its ball head 12 into the cage 4 with elastic widening of the segments 42. So these parts are captive pre-assembled. After that, the pressure element 5 can be inserted a few turns into the internal thread 31. In this case, the front part of the slot 35 remains free with the extension 36. The fixation arrangement is now ready for use.

To attach the implant, first the bone screw 1 is tightened by means of a guided through the sleeve 3 to the star-shaped receptacle 14 in the ball head 12 wrench. In the next step, the rod 7 can be inserted into the transverse bore 35 for assembly. Finally, the pressure element 5 is screwed in and tightened with a wrench inserted into the recess 54. Here, the generated compressive forces are passed over the cross-sectional bar 7 on the end face 43 of the clamping cage 4, whereby it is pressed forward in the axial direction. The resulting compressive forces are converted due to the taper in the interface between the outer shell 40 of the clamping cage 4 and the fit 32 in combination with the constriction 33 inwardly acting pressing forces that are much higher in terms of amount and the spherical screw head 12 fixed to the Press inside of the segments 42 of the clamping cage 4. This achieves an angle-stable fixation.

The dashed line in FIG. 2 shows a possible range for the different angles, which can take the polyaxial bearing of the screw 1 with respect to the holder 3.

A variant of the first embodiment is in the Fig. 10 and 11 shown. Similar parts bear the same reference numerals. The main difference lies in the execution of the Sleeve 8 and the clamping cage 9. The sleeve 8 is executed in a hollow cylinder-like manner with an inner bore 80 running from a rear end to the front end, having an internal thread 81 for receiving the pressure element 5 in its rear region and a segment 82 shaped like a hollow sphere segment in its front region 'a snug fit 82 provided. In the intermediate region, a substantially smooth inner wall is provided, against which the clamping cage 9 abuts with its jacket 90. The hollow spherical segment-shaped segment 82 'of the fitting seat 82 is formed complementary to the ball head 12 of the bone screw 12. The mouth at the front end of the sleeve 8 is narrowed and passes the shaft 10, but prevents the ball head 12 at a passage.

The sleeve 8 further comprises two diametrically opposed longitudinal slots 85. They extend from the rear end of the sleeve 8 over the entire region of the internal thread 81 to the fit 82. Offset from the longitudinal slots 85, two window-like recesses 88 are arranged.

The tensioning cage 9 has a jacket 90, which is provided with a preferably conical outer surface, and an inner space 94. Its width is adjusted to the width of the ball head 12 such that a fit results over an area. This area forms a second part 82 "of the mating seat 82. It cooperates with the hollow spherical part 82 'so that a continuous fit 82 is formed to receive the ball head 12. The joint between the two parts 82' and 82" lies in the area largest expanse and thus where the equator 13 of the bone screw lies.

Slit pairs 91 are arranged opposite one another in the rear region of the tensioning cage, resulting in two diametrically arranged tongues 98. In the inserted state of the clamping cage 9, they spread and hold in the window-like recesses 88, whereby the clamping cage 9 prevented from falling out of the sleeve 8 is. Thus, a preassembled unit of inserted into the sleeve 8 bone screw together with clamping cage 9 are formed. At its rear end face of the clamping cage 9 is provided with a groove 97. It is preferably formed semi-cylindrical and serves for the positive reception of a holding rod of an implant. 7

Reference will now be made to the embodiment of the invention. How out Fig. 4 1, the fixation arrangement comprises a holder 2 'for a bone screw 1', which has a shaft 10 'with a bone thread 11' arranged in the front region. Towards the rear end, the shaft 10 'has an increasing thickening 15'. It extends to a head 12 ', which has a recess 14' for receiving a wrench (not shown).

The holder 2 'comprises a base plate 3' and a clamping sleeve 4 '. The base plate 3 'has on its front side a bearing surface 37' for engagement with a bone or other tissue parts. It may be provided with optional retaining pins 38 '. The base plate 3 'is traversed by a holding opening 30', which is widened in its central area in a spherical cap to a fit 32 '. In these, the clamping sleeve 4 ', which in turn has a spherical shell 40' with suitable dimensions added. By the complementary surfaces of the spherical shell 40 'on the one hand and the spherical cap-shaped fit 32' on the other hand, the clamping sleeve 4 'is pivotally mounted. In this condition, the fixation assembly is pre-assembled and ready for implantation.

The clamping sleeve 4 'has a central opening 44' through which the bone screw 1 'with its shaft 10' is inserted during implantation. The opening 44 'has a substantially cylindrical shape, with an inwardly facing radial bar 46' at the front end (s. Fig. 7a ). Further embodiments for the opening 44 ', 44 "are in Fig. 7b and c, namely, with a complex shaped ledge 46 "on which one or two turns of internal thread 47 'are arranged, or a pilotless conical forwardly tapered bore, but alternatively, the aperture may be cylindrical as well The clamping sleeve 4 'is preferably slotted, with a plurality of slots 41' open at one side to the front (see FIG. Fig. 6a ). But it can also be a continuous slot 42 'may be provided (s. Fig. 6b ).

The thickening 15 'extends over a region which is about two to three times as long as the length of the clamping sleeve 4' determined by the length of the opening 44 '. When screwing the screw 1 ', this moves axially forward with its shaft 10' through the base plate 3 '(symbolized by the single arrow in Fig. 5 ). The conical thickening 15 'comes into contact with the inner wall of the opening 44', and spreads the clamping sleeve during the further axial movement (see double arrow in FIG Fig. 5 ). As a result, the clamping sleeve 4 'with its spherical shell 40' is forced against the spherical cap-shaped interference fit 32. Due to the wedge effect, considerable forces arise due to the small cone angle of less than 10 degrees, so that effective compression between the clamping sleeve 4 'and the interference fit 32' of the base plate 3 'results. The screw 1 'is thus fixed in its respective pivoted angular position relative to the base plate 3'.

The thickening 15 'may be conical as described above. This is in Fig. 8a shown. A non-inventive embodiment is in Fig. 8c shown. According to the invention, the thickening 15 'is provided with a secondary thread 16' ( Fig. 8b ). This is more frequent for easier searching into the internal thread 47 'on the clamping sleeve 4' and preferably has the same pitch as the thread 11 'on the shaft 10'. This allows the axial force and thus the spreading and pressing action can be increased. In addition, the protection against unintentional loosening by turning back the screw 1 'in the bone improves. In a further alternative embodiment, the thickening 15 "is displaced further to the rear end and has a forward-facing collar surface 17 ', so that the conical region of the thickening can be made shorter, so that a structural integration with the head 12' is made possible. This results in a bone screw 1 'which is particularly easy to produce. Preferably, the last alternative is used together with a type of clamping sleeve 4', as used in US Pat Fig. 7c is shown. It allows thanks to their also conical opening 44 ", which is preferably complementary to the taper of the thickening 15", a special compact unit.

A variant of the embodiment according to the invention is in FIGS. 12 and 13 shown. Similar parts are provided with the same reference numbers. The bone screw 1 "has a shaft 10" with a conical bone thread 11 "arranged in the front region, and at the rear end the shaft 10" has a thickening 15 "', to which the head 12" adjoins. The head 12 "is designed to broaden rearwardly from the thickening 15"'. The thickening 15 "'comprises a conical front portion in the region of the transition to the shank 10" and a sheath in the rear portion on which a secondary thread 16 "is formed and has a pitch which is slightly less than half the pitch of the bone thread 11 "(for example, 7/15).
A variant of the clamping sleeve 9 'is in Fig. 13 shown. It is different from the ones in Fig. 7 illustrated clamping sleeves 4 'substantially by a complex configuration of the through hole 94'. This includes a shorter front cylindrical portion 94 "'and a longer rearward conical portion 94", at the junction of which an inwardly projecting radial ledge 96 'is arranged with a short internal thread 97'. By "short" is meant here up to two revolutions.

In this variant, the bone screw 1 "widened with its thickening 15" 'the cylindrical portion 94' 'of the complex passage opening, under a feed force, which is generated by the in the short internal thread 97' engaging secondary thread 16 "when screwing. The conical shape of the head 12 "together with the conical part 94" ensures a further spreading and clamping of the bone screw 1 ", so that it is secured against unintentional loosening.

The holder can also be designed so that several bone screws are held. An example of this is in Fig. 9 shown. Shown is a multiple holder 3 ", which has two receiving seats to additionally record a second clamping sleeve 4".

Claims (16)

1. Fixation apparatus comprising a bone screw (1') and a holder (2'), wherein the holder (2') has a base plate (3') and a tension sleeve (4', 9') receiving the bone screw (1'), wherein the bone screw (1') comprises a shaft (10') with a front thread (11') and a head (12') at the rear end, wherein the base plate (3') comprises a through borehole (30') and a receiving seat (32'), in which the tension sleeve (4', 9') is mounted and able to swivel, the shaft (10') has a thickening (15') in a rear region, forward of the head (12'), whose width increases toward the head (12'), and the opening width of the tension sleeve (4', 9') is larger than the width of the shaft (10') immediately before the thickening (15') and smaller than the greatest width of the thickening (15'),
   characterized in that
   a secondary thread (16', 16") is configured on the thickening (15', 15"'), which has a smaller pitch than the thread (11') on the shaft (10').
2. Fixation apparatus according to Claim 1,
   characterized in that the thickening (15') has a cone angle of at most 15 degrees, preferably at most 5 degrees, and/or a greater length than a length of the tension sleeve (4', 9'), preferably at least two and a half times.
3. Fixation apparatus according to Claim 1 or 2,
   characterized in that the secondary thread (16', 16") is multiple thread.
4. Fixation apparatus according to Claims 1 to 3,
   characterized in that the thickening (15') has a conical front segment and an envelope surface on which the secondary thread (16"') is formed.
5. Fixation apparatus according to Claims 1 or 4,
   characterized in that the pitch of the secondary thread (16") is 0.4 to 0.6 times the pitch of the front thread (11") on the shaft (10').
6. Fixation apparatus according to one of Claims 1 to 5, characterized in that the thickening (15") is combined with the head (12'), and the head (12") preferably broadens conically toward a rear end.
7. Fixation apparatus according to one of Claim 1 to 6,
   characterized in that the thickening (15") has a front flange (17').
8. Fixation apparatus according to one of Claims 1 to 7, characterized in that a boundary surface between the tension sleeve (4', 9') and the receiving seat (32') is ball-shaped, in particular, an outer envelope (40', 90') of the tension sleeve (4', 9') being spherical.
9. Fixation apparatus according to one of Claims 1 to 8, characterized in that the receiving seat (32') is open to a rear end and forms a mouth whose width is preferably smaller than a width of a middle region of the receiving seat (32').
10. Fixation apparatus according to one of Claims 1 to 9, characterized in that the tension sleeve (4') is slotted.
11. Fixation apparatus according to Claim 10,
    characterized in that a continuous slot (42') running the entire length of the tension sleeve (4') is provided.
12. Fixation apparatus according to Claim 10,
    characterized in that a plurality of one-ended slots (41'), preferably open toward a front end, are provided.
13. Fixation apparatus according to one of Claims 1 to 12, characterized in that a preferably cylindrical inner wall of the tension sleeve (4', 9') has an encircling, inwardly facing radial strip (46', 96').
14. Fixation apparatus according to Claim 13,
    characterized in that an inner thread (47', 97') is arranged on the radial strip (46', 96').
15. Fixation apparatus according to one of Claims 1 to 14, characterized in that a continuous opening (94') of the tension sleeve (9') comprises a plurality of segments, a cylindrical segment (94"') and a conical segment (94"), wherein preferably at the transition between the segments there is arranged a radial strip (96') with, further preferably, a short internal thread (97').
16. Fixation apparatus according to one of Claims 1 to 15, characterized in that the holder is fashioned as a multiple holder (3") with at least two receiving seats for tension sleeves (4', 4", 9').

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